Amateur astronomers are discovering pieces of a giant comet that broke apart
in antiquity as the fragments zoom perilously close to the Sun.

July 7, 2000 -- In October 1965 comet Ikeya-Seki swooped past the Sun barely
450 thousand kilometers above our star's bubbling, fiery surface. Gas and
dust exploded away from the comet's core as fierce solar radiation vaporized
the icy nucleus. Most comets wouldn't survive passing as close to the Sun as
the Moon is to the Earth, but Ikeya-Seki literally came through with flying
colors. When the comet emerged from perihelion (closest approach to the Sun)
it was so bright that observers on the street with very clear skies could
see it during broad daylight if the Sun was hidden behind a house or even an
outstretched hand.

"In Japan (where observers spied the comet 1/2 degree from the Sun) it was
described as 10 times brighter than the Full Moon," recounted Brian Marsden
of the Harvard Center for Astrophysics in the December 1965 issue of Sky &
Telescope. "At Kitt Peak National Observatory in Arizona, Stephen Maran
observed the comet with binoculars from within the shadow of a black disk
erected to hide the Sun. '[It was] the most splendid thing I have ever
seen,' he noted."

Ikeya-Seki, a.k.a. "The Great Comet of 1965", is a member of the family of
comets called Kreutz sungrazers (after the nineteenth-century German
astronomer who studied them in some detail). These ill-fated visitors to the
inner solar system have been seen to pass less than 50,000 km above the
Sun's photosphere. Most never make it past perihelion -- they are completely
obliterated. But the few that do, like Ikeya-Seki, can be very bright.

"There are 2 or 3 really bright ones like Ikeya-Seki every century," says
Brian Marsden. "Most of these sungrazers are fragments from the breakup of a
giant comet at least 2000 years ago, perhaps the one that the Greek
astronomer Ephorus saw in 372 BC. Ephorus reported that the comet split in
two. This can be made to fit with my calculation that Ikeya-Seki and an even
better Kreutz sungrazer observed in 1882 split off from each other when
their parent revisited the Sun around AD 1100. Splits have occurred again
and again, producing the sungrazer family, all still coming from the same
direction."

The nucleus of Ikeya-Seki was probably some kilometers across. Tinier pieces
of Ephorus's comet streak past the Sun every day. Measuring perhaps only ten
meters in diameter, they brighten briefly as they approach the Sun and
disappear forever when they vaporize above the photosphere. Most of the
faint fragments must have escaped detection entirely.

Now, thanks to coronagraphs on board the orbiting ESA/NASA Solar and
Heliospheric Observatory (SOHO), amateur and professional astronomers can
easily monitor the sky around the Sun for the telltale streaks of faint
sungrazers. All that's needed is a computer and a connection to the
internet.

"In late1998 we put SOHO's realtime coronagraph movies online so that anyone
with an internet connection could access the data" says Doug Biesecker, a
solar physicist at the Goddard Space Flight Center and SOHO's champion comet
hunter with 47 finds. "Over a three year period before that time we had
found 58 comets near the Sun in SOHO images. Now the total is up to nearly
170. Amateur astronomers watching coronagraph movies on the web are
responsible for nearly all of the new finds this year. They're keeping me
very busy!"

One of the most successful amateur comet hunters is Michael Boschat. He's
credited (or shares credit) with a dozen discoveries since March 2000.

"I use the C3 512 x 512 pixel images," explains Boschat. "They appear on the
SOHO site every 30 minutes and I download them as soon as they do. After I
have four images I begin to loop them using GIF animation software that can
be found on the Internet. I usually loop them at four frames per second
looking for an object that is moving towards the Sun in a steady manner. I
also use a magnifying glass to watch the possible comet move. After I feel
it is a comet I put my mouse arrow as near as possible to the object to get
the X and Y coordinates then send all that information off via email to
Douglas Biesecker at Goddard."

All of the comets identified in images from SOHO are called "comet SOHO"
followed by a number denoting the order of discovery. This differs from the
traditional convention of naming a comet after the person who finds it. The
most recent confirmed sungrazer, as of July 4, 2000, was comet SOHO-143. The
International Astronomical Union's official designation for SOHO-143 is
C/1998 K15, because the actual images were obtained in 1998, with the K15
indicating that this was the fifteenth comet (of any description) found
during the second half of May.

"It started in the early 1980s with the SOLWIND mission, which also carried
a coronagraph," explains Biesecker. "SOLWIND detected 6 sungrazers and they
were all named after the satellite. The tradition continued for the Solar
Maximum Mission (10 comets) and now for SOHO (143 confirmed comets and
counting). It's reasonable because all of the comet finds have to be
confirmed by mission scientists who are familiar with the hardware. Cosmic
rays, noise in the detectors and other factors can mimic comets and we have
to carefully examine each one. It's really a team effort."

"In the early 1980s there were also the 'IRAS' comets, found by the Infrared
Astronomical Satellite," added Marsden. "Most of the comets found nowadays
from the ground--and far from the sun--are named 'LINEAR', acronym for the
Lincoln Near Earth Asteroid Research team, which scans the sky intensely
with a U.S. Air Force telescope." (One of these is about to become a naked
eye object in late July, 2000.)

Biesecker says he hopes the recent spate of amateur discoveries will
continue unabated.

Note: An asterisk denotes a professional member of the
SOHO team. Others are amateur astronomers.

"The amateur discoveries are important because they can help us understand
the fragmentation history of Kreutz sungrazers by monitoring the numbers and
brightness of the smallest ones that we can see with the SOHO coronagraphs.
The amateurs are also finding a few unrelated 'near-Sun' comets [this is not
an official name] that pass within 10 to 20 solar radii of the Sun. This is
an under-sampled population of comets."

If you're interested in joining the hunt for sungrazing comets, a good place
to start is the Solar and Heliospheric Observatory's realtime images web
page where coronagraph data are posted every 30 minutes, and sometimes even
more frequently. Data from the satellite are available to the general public
at the same time as to the scientific community. If you think you've found
something, first review the basic criteria for a discovery before forwarding
the details to scientists at the Goddard Space Flight Center. Confirmed
finds are posted daily on the Whats'New" area.

SOHO (the Solar and Heliospheric Observatory) is a mission of international
cooperation between NASA and the European Space Agency. It is managed by the
Goddard Space Flight Center for the NASA HQ office of Space Science.